Differential pulse code modulation (DPCM) of digitized video signals is often an important step in the digital transmission of frame sequences of television images. In DPCM each sample descriptive of a picture element ("pixel") in a raster scanning of successive frames is differentially combined with a related picture element and the resulting difference is coded in a pulse coding modulation procedure. DPCM procedures where a pixel is differentially combined with the preceding sample, or a sample one scan line earlier, or some other spatially adjacent sample are known and can be classified as being intraframe DPCM procedures. However DPCM of the "interframe" type where a pixel is differentially combined with the correspondingly located pixel in the previous frame, has particular advantages over intraframe DPCM procedures. If run length coding is done, long runs of zero-value DPCM samples are likely to occur in static portions of a sequence of frames. Quantizing noise is temporal in nature in interframe DPCM rather than being spatial in nature as in intraframe DPCM; and the human visual system is less likely to discern temporal quantizing noise, owing to time-integrating properties in the visual system. Error propagation in interframe DPCM coding tends to affect single pixels; in intraframe DPCM an error may propagate over extensive portions of a frame and be very intrusive insofar as the human visual system is concerned. Interframe DPCM transmission of video signals is sometimes preceded by procedures for selectively filtering the video signals into respective components.
The structure for an interframe digital pulse code modulator is generally as follows. The current video signal or video signal component is digitized to form a stream of digital input samples that is supplied to the minuend input port of a subtractor. The subtrahend input port of the subtractor receives predicted values for the digital input samples, and the difference output port responds to the samples received at its input ports to generate a digital error signal. This digital error signal is supplied to a quantizer, which classifies the digital error signal into range bins. Often the modulator is of a type in which the range of sample values for each bin in the quantizer can be adjusted during operation. The quantizer may define uniform range bins simply by suppressing less significant bits in the error signal or may be of a type defining different size range bins. The quantizer output signal, a stream of range bin numbers symmetrically disposed about zero, is the modulator output signal. The predicted values for the next frame of samples are obtained as follows. Each sample of the modulator output signal comprising a succession of bin numbers is converted to the nominal error signal sample value for that bin number and is added to the predicted value of the current digital input sample to arrive at the predicted value of the digital input sample to occur one frame later. This latter predicted value is then written into a frame store memory to be read a frame later to the subtractor subtrahend input port.
The structure for an interframe digital pulse code demodulator is generally as follows. Each sample of the demodulator input signal, which corresponds to a sample of the modulator output signal described in the previous paragraph, is a bin number which is converted to the nominal error signal value for that bin number and is furnished to a first addend input port of an adder. A second addend port of the adder receives a predicted value, and a digital signal similar to the digital input signal supplied to the digital pulse code modulator is generated at the sum output port of the adder. This digital signal is written into a frame store memory to be read a frame later to the second addend port of the adder as a predicted value.
N. J. Fedele, A. Acampora, P. J. Burt and R. Hingorani in U.S. Pat. No. 4,663,660 issued May 5, 1987 and entitled "COMPRESSED QUANTIZED IMAGE-DATA TRANSMISSION TECHNIQUE SUITABLE FOR USE IN TELECONFERENCING" describe spatial frequency analysis of television images being done on a per octave basis within the confines of an interframe DPCM feedback loop. The spatial frequency analysis results are individually quantized and symbol coded in respective circuitry, and the symbol codes are time-division multiplexed (TDM'd) for transmission over a digital link having narrow bandwidth compared to digital links that would be required for transmitting digitized video samples. The symbol coding technique described by Fedele et al. uses statistical coding, such as Huffman coding, of DPCM samples with non-zero values and of run-lengths for zero-value DPCM samples.
L. N. Schiff in U.S. patent application Ser. No. 040,470 filed Apr. 17, 1987, assigned to RCA Corporation and entitled "TWO RESOLUTION LEVEL DPCM SYSTEM" describes spatial frequency analysis of television images being followed by respective differential pulse code modulators for each analysis result. The DPCM signals are individually symbol coded, and the symbol codes are TDM'd for transmission over a narrow bandwidth digital link.
The problem with the systems described above is that the time-division multiplexing of the symbol code is complex since the rates of the symbol codes being multiplexed are non-uniform. Further, these systems are unduly complex in structure, each requiring a plurality of symbol encoders, for example.